The electrical properties and physical configuration of the cochlear electrode array define and delimit the stimulation strategies that can be employed to activate the auditory nerve fibers. Any attempt to predict or measure the response of these neurons to a given stimulus must start with an assumed electrode design. However, all currently available cochlear electrodes are based on designs determined primarily by the properties of commonly available materials and fabrication processes, rather than on the basis of fundamental properties of neurons and cochlear geometry. Furthermore, clinical results with cochlear prostheses to date suggest that many of the original premises regarding optimal electrode designs may require modification. The purpose of this project is to develop and characterize electrode-contact materials, to custom fabricate intracochlear electrodes for use in animal experiments and bench tests, and to design new electrodes that should lead to improved prosthetic function. Specific tasks include: 1. To characterize existing clinical electrode arrays. 2. To explore the interfacial properties of the metal/electrolyte junction, including fabrication and characterization of new materials such as activated iridium that promise to greatly extend the available impedance properties of this interface. 3. To fabricate multicontact electrode arrays for acute and chronic animal experiments that provide for a variety of accurately positioned current sources. 4. To develop and test generalized mathematical models of electrode interfaces and geometries, using field-mapping techniques in tank experiments. 5. To design improved electrode arrays for human use, based on the findings of this and other projects of the research program, and to build prototypes of these arrays for evaluation in animal studies. In addition to optimizing electrical properties, the electrode designs that emerge from this project will consider issues such as surgical handling properties and economy of manufacture, to assure that they can be transferred readily to manufacturers of clinical auditory prostheses.